A number of lines of evidence indicate that radon gas (progeny) is a potent lung carcinogen. Indoor radon has recently been reported to be a serious health risk in the U.S. Advisories have been issued indicating it is the second leading risk factor for lung cancer and may be responsible for up to 20,000 lung cancer deaths per year. While radon might be presumed to be a carcinogen in the home environment, it is unclear whether actual empirical evidence will show an increased lung cancer incidence in those non-occupationally but residentially exposed. A population-based case-control study is proposed to evaluate the association between residential radon exposure and the incidence of lung cancer among females in the state of Iowa. Because of the population composition of Iowa, over 98% of the cases will be of the white race. This 5-year study will involve 600 cases and 600 controls between the ages of 40 and 84. It will have three major components: 1) a cage-control mailed/telephone follow-up questionnaire; 2) a residential radon exposure assessment; and 3) independent histopathologic review of lung cancer tissues. Eligibility criteria will include the requirement of 25 years or more of residence in the same home at time of initial diagnosis among cases and at time of initial contact among controls. Our data collection goals will be to obtain at least an 80% response rate from the control group and an 85% response rate from the case group. Recent previous experience indicates these goals are attainable. Although several epidemiologic studies of radon in the home and lung cancer have already been performed or are currently being performed, this study is better designed to address this important health topic for several reasons: 1) a substantial proportion of the population will reside for 25 years or more in the same home; 2) a high quality, statewide cancer surveillance system which will allow for rapid identification of newly diagnosed lung cancer cases; 3) 70% of homes in the state have radon levels exceeding 4 pCi/liter and 4) multiple year-long measurements of radon in a subject's home will yield more accurate estimates of residential radon exposure. All of these reasons are described in further detail within this proposal.